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Superimposition of Strong Phase-Locked Microwave Signal on Laser Modulation Current at Twice the Bit Frequency

Publishing Venue

IBM

Related People

Olsen, CM: AUTHOR

Abstract

Fig. 1 also indicates how a phase-locked AMS, whose frequency, f sub AMS, is two times the data frequency, f sub data, can be generated by appropriate multiplication of the transmitter reference clock frequency, f sub clk.

Country

United States

Language

English (United States)

This text was extracted from an ASCII text file.

This is the abbreviated version, containing approximately
52% of the total text.

Superimposition of Strong Phase-Locked Microwave Signal
on Laser
Modulation Current at Twice the Bit Frequency

Fig. 1 also
indicates how a phase-locked AMS, whose frequency,
f sub AMS, is two times the data frequency, f sub data, can be
generated by appropriate multiplication of the transmitter reference
clock frequency, f sub clk.

Disclosed is
a proposal to superimpose a phase-locked Auxiliary
Microwave Signal (AMS) on the regular DATA laser modulation signal at
two times the bit frequency. The
phase-locked AMS technique is of
particular relevance to Multi-Mode Fiber (MMF) data links operating
in the > 1 Gbps regime which, in certain cases, may need extra
protection against modal noise. By
phase-locking the AMS to the DATA
signal, an excessive jitter on the receiver output is avoided, any
intermodulation penalty is eliminated, and the general desire of
controlling the AMS frequency and phase is facilitated. Furthermore,
that the AMS frequency is only twice the bit frequency makes it
feasible to place the AMS driver chip outside of the laser package,
to manufacture the AMS oscillator/driver chip in the same technology
as the DATA driver chip, and to use the same laser package (e.g., a
TO can) as for conventional laser modulation.

The general
idea of superimposed microwave modulation was
originally proposed by (1) and refined by (2).
However, their
approach was geared toward low-speed long-distance MMF systems. In
(3) the technique was extended to Gbps short-distance data links,
though utilizing a free-running microwave signal.

The presence
of the AMS significantly degrades the laser
coherence properties. Typically, the
laser coherence length is
reduced by 2-3 orders of magnitude. The
laser spectrum is
drastically broadened and becomes very insensitive to external
reflections. Theoretically, it was found
the laser mode partition
noise k-factor is reduced by more than 50%.
All three effects are
contributing to reducing the modal noise, both the high-frequency and
low-frequency components. For efficient
modal noise suppression, the
AMS must have an amplitude, which is around 150% of the peak DATA
modulation current (exclusive the threshold current) and the AMS must
have a frequency, fAMS, which is between 50-100% of the on-level
relaxation frequency, frfr is typically in the range of 3-6 GHz.

The
phase-locked AMS technique is schematically shown in
Fig. 1. The AMS and the DATA signal are
simply added together. The
AMS is characterized by three values: the modulation index, mAMS, the
frequency, fAMS, and the phase AMS mAMS is the relative difference
between the AMS amplitude current and the peak DATA modulation
current (exclusive the threshold current) and AMS is the phase of the
AMS with respect to the 50% transition
times of the DATA modulation
signal.